1. Field of the Invention
This invention relates to the annealing of PTC conductive polymer compositions.
2. Summary of the Prior Art
PTC conductive polymer compositions are known for use in self-limiting strip heaters and in other electrical devices; such compositions can contain two crystalline polymers having substantially different melting points. It is also known to anneal PTC compositions, after they have been shaped, in order to reduce their resistivity, by heating them for extended period, e.g. of several hours, at a temperature above the melting point of the composition. Reference may be made for example to U.S. Pat. Nos. 3,793,716, 3,823,217, (Kampe), 3,861,029 (Smith-Johannsen et al), 3,914,363 (Bedard et al) and 4,177,376 (Horsma et al) and to commonly assigned U.S. patent applications Ser. Nos. 84,352 (Horsma et al), 88344 (Lutz) and the continuation-in-part thereof (MPO701) Ser. No. 134,354 732,792 (Van Konynenburg et al), now abandoned 750,149, (Kamath et al), now abandoned, 751,095 (Toy et al), now abandoned, 798,154 (Horsma), now abandoned, 965,343 (Van Konynenburg et al), now U.S. Pat. No. 4,237,441, 965,344 (Middleman et al), now U.S. Pat. No. 4,238,812, 965,345 (Middleman et al), now abandoned, and 75,413 (Van Konynenburg) and the eight applications filed Apr. 21, 1980 by Gotcher et al (MPO712, 157/111) Ser. No. 141,984, Middleman et al (MPO713, 157/112) Ser. No. 141,987, Fouts et al (MPO714, 157/113) Ser. No. 141,988, Evans (MPO715, 157/114) Ser. No. 141,989, Walty (MPO719, 157/161) Ser. No. 141,990, Fouts et al (MPO720, 157/162) Ser. No. 141,991, Middleman et al (MPO724, 157/167) Ser. No. 153,053 and Middleman et al (MPO725, 157/168) Ser. No. 142,054. The disclosure of each of these patents and applications is incorporated herein by reference.
I have discovered that when a PTC composition containing a mixture of two crystalline polymers of different melting points is annealed at a temperature between the two melting points, the annealed composition has improved electrical properties as compared to a composition annealed at a temperature above the higher melting point, as recommended by the prior art. The improved electrical properties can for example be improved resistance stability and/or linearity ratio when the composition is heated externally and/or when it is heated internally by passing current through it, for extended periods, e.g for 1000 hours or more.
In one aspect, therefore, this invention provides a method of modifying the electrical characteristics of an electrical device comprising a PTC element composed of a conductive polymer composition which exhibits PTC behavior and which comprises
(i) a polymer component which comprises a mixture of a first crystalline polymer having a first melting point T.sub.1 and second crystalline polymer having a second melting point T.sub.2 which is at least (T.sub.1 +25).degree.C., and PA1 (ii) a particulate filler component which has been dispersed in said polymer component and which comprises a conductive filler; which method comprises annealing said device at a temperature T.sub.A which is between T.sub.1 and T.sub.2 for a time sufficient to reduce the resistivity at 25.degree. C. of said conductive polymer composition from a first value, .rho..sub.o, prior to said annealing to a second value, .rho..sub.A, after said annealing, where .rho..sub.A is less than 0.8.times..rho..sub.o.